The advent of affordable natural gas has been embraced by the makers of over the road vehicle engines and they are now adapting their formerly diesel fueled engines to operate with natural gas. This requires some changes, including the addition of a throttle in the intake air stream. When the throttle is closed quickly a condition call surge can be initiated. When a turbocharger is in a state of surge it no longer can effectively compress the intake charge, however it is still absorbing power from the exhaust gas flow. In a throttle closing transient this loss of compression causes the turbocharger to speed up, while simultaneously the flow rate of air into the engine is decreased. In a fraction of a second the exhaust power available to the turbocharger decreases dramatically, slowing the turbocharger back down and in turn increasing the compression pressure.
This unstable operation can occur for several oscillations, causing the vehicle to vibrate and the torque output by the engine to vary. Thus, there is a need for improved designs to control the turbocharger operation during the transient throttle closing period in a natural gas powered engine.